1. Field of the Invention
The present invention relates generally to a process and apparatus for prolonging the useful life of catalyst for continuous use in a high temperature fluidized catalytic cracking environment and more specifically to an apparatus and process for controlling and reducing the temperature of catalyst residing in the stripping zone to improve catalytic cracking efficiencies, extend time between catalyst replacement, and lower the environmental impact of the hydrocarbon cracking process.
2. Description of Related Art
Fluidized catalytic cracking (“FCC”) processes are widely used for the conversion of hydrocarbon feed streams, such as vacuum gas oils and other relatively heavy oils, into lighter and more valuable hydrocarbon products.
The basic components of the fluidized catalytic cracking unit (“FCCU”) include a reactor, a disengager, a catalyst stripper and a catalyst regenerator. The reactor includes a contact zone, most often in a riser column, where the hydrocarbon feed is contacted with hot particulate catalyst to crack the hydrocarbon feed into lighter hydrocarbon products. A separation zone, or disengager, where product vapors from the cracking reaction are separated from the catalyst, most often is located at, or is contiguous with, the downstream end of the riser column.
The FCC process is carried out by contacting the hydrocarbon feed with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport.
In riser cracking, regenerated catalyst and starting hydrocarbon feed enter a generally cylindrical reactor and are transported upward by the expansion of the gases that result from vaporization of the hydrocarbons and fluidizing media upon contact with the hot catalyst and, optionally, by provision of a lift gas. At the end of the reaction zone, the reactor effluent comprises spent catalyst, hydrocarbon product and optional lift gas. In order to recover the catalyst and the hydrocarbon products, the effluent is separated in a separation zone into a hydrocarbon rich vapor phase and a solids rich phase comprising spent catalyst.
The separated, still hot, spent catalyst is delivered from the separation zone to the stripper zone. During the cracking phase, the particulate catalyst absorbs hydrocarbon vapors. Spent catalyst, therefore, contains an appreciable quantity of removable or strippable hydrocarbon. The catalyst stripper receives spent catalyst from the separation zone and removes entrained hydrocarbons from the spent catalyst by counter-current contact with steam or other stripping medium.
In the stripping zone, a stripping gas, most often steam, is introduced via an inlet in a lower portion of the stripping vessel. The stripping gas rises through the stripping vessel and flows counter current to the hot spent catalyst flowing downward, across mixing baffles or packing, toward an outlet located at the lower portion of the stripper zone. As the stripping gas flows through the hot spent catalyst, it strips the hydrocarbon vapors absorbed on the catalyst surface. The stripped hydrocarbon vapors then may be recovered.
The hot spent stripped catalyst then is transferred from the stripper to a catalyst regenerator for purposes of removing the coke. During the cracking reaction, coke is deposited on the catalyst, which partially deactivates the catalyst. Coke is a byproduct of the cracking reaction and is comprised of hydrogen and carbon and possibly other materials in trace quantities, such as sulfur and metals, which enter the process with the starting material.
Coke is removed from the catalyst in the regenerator by oxidation with an oxygen-containing gas. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which leaves the regenerator with gaseous products of coke oxidation, generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst as it is returned to the reaction zone for another cycle through the FCCU.
In the above-described fashion, the fluidized catalyst is circulated continuously from the reaction zone to stripper to the regeneration zone and then back again to the reaction zone for the beginning of another cycle. The hydrocarbon products produced by the FCC reaction are recovered in vapor form and transferred to downstream product recovery facilities. Specific details of the aforementioned various contact zones, regeneration zones, and stripping zones along with all the known arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
Current industry advances in fluidized catalytic cracking allow the use of heavier hydrocarbon feeds (ie., with higher Conradson Carbon values). The heavier hydrocarbon feeds generally cause an increase in the amount of coke deposited on the catalyst during the catalytic reaction. As previously described, the heat required for the endothermic cracking reaction conditions in the reactor is provided by the burning of the coke from the spent catalyst. Accordingly, the use of heavier feeds can lead to excess heat generated during catalyst regeneration due to the burning of larger amounts of coke developed on the catalyst. The additional heat produced by the heavier feeds can create a number of problems, including upsetting the heat balance, damaging the equipment, damaging the catalyst and limiting the amount of hot catalyst that can be fed to the reaction, all of which ultimately result in lower product yields.
Some of the recent advances for cracking heavier feedstocks include employing higher temperatures in the cracking process in order to produce selectively light olefins. Regenerator temperatures today approach 1700° F. and cracking conditions may approach 1500° F. in some processes. This expedient, however, can cause deactivation of the catalyst, resulting in a shortened useful life of the catalyst, decreased efficiency of the cracking process as well as increased maintenance expenditures, FCCU down time and environmental impact. Under these recent high-temperature cracking conditions, catalyst may be maintained at relatively high temperatures in the presence of steam throughout the entire cracking process, including in the stripping zone, which may cause catalyst deactivation.
It would therefore represent a notable advance in the state of the art if an apparatus and process could be developed that reduced the temperature of the catalyst at appropriate phases of the cracking process to preserve valuable resources, maintain optimum efficiency of the furnace equipment and operations, reduce down time and lessen any impact on the environment.
Most, if not all, of the prior art methods of removing heat during the cracking process have focused on the hottest stage of the process, the regeneration stage. A very common method for heat reduction in the regeneration stage uses heat exchange means through indirect contact with a cooling medium, i.e., a catalyst cooler attached to the regenerator. Generally, indirect contact heat exchange means use cooling coils or tubes, through which a cooling fluid is passed. The cooling coils can run through a bed of the catalyst particles internal to the regenerator or through a separate catalyst bed external to the regenerator.
Heat exchangers utilizing cooling coils or tubes running through a fluidized catalyst particle bed internal to the regenerator are illustratively shown in U.S. Pat. No. 4,009,121 to Luckenbach, U.S. Pat. No. 4,220,622 to Kelley, U.S. Pat. No. 4,388,218 to Rowe and U.S. Pat. No. 4,343,634 to Davis. These internal exchangers are effective, but offer less precise temperature controlling ability, tend to have shortened life expectancy and are difficult to retrofit and/or service when they are exposed to and located in the extreme environment of the regenerator.
External heat exchangers are generally external flow-through type coolers where catalyst is withdrawn from the regenerator and directed into a separate vessel having cooling tubes or coils therein. Generally, flow-through coolers are either gravity feed, where catalyst enters one upper inlet and exits a lower outlet, or they employ fluidized transport that moves catalyst from a lower inlet past the cooling coils to an upper outlet. Back-mix heat exchangers are shown in U.S. Pat. No. 3,672,069 to Reh et al and U.S. Pat. No. 4,439,533 and U.S. Pat. No. 4,483,276 both to Lomas et al, and U.S. Pat. No. 5,027,893 to Cetinkaya et al. relating to a heat exchanger with a combination of back-mix and flow-through characteristics. Other variations, illustratively shown in U.S. Pat. No. 2,735,802 to Jahnig and U.S. Pat. No. 4,615,992 to Murphy, disclose a hot catalyst inlet at the mid-portion of the heat exchanger and an outlet at the bottom of the heat exchanger where a fluidizing gas moves the cooled catalyst back up to the regenerator vessel. Other flow-through heat exchangers are placed between the hot catalyst source (regenerator) and the reaction zone to regulate the temperature of the catalyst entering the reaction. Examples of such a system are found in U.S. Pat. Nos. 4,284,494 and 4,325,817 to Bartholic et al. A regenerator apparatus using pure gravity feed flow-through heat exchanger is shown in U.S. Pat. No. 2,970,117 to Harper. The Harper heat exchanger removes catalyst from the catalyst bed of a single stage regeneration vessel and returns the cooled catalyst at a lower portion of the catalyst bed. A regenerator apparatus using fluidized transport to move catalyst from the bottom of a single stage regenerator upward over the cooling coils and back to the top of the regenerator is described in U.S. Pat. No. 4,064,039 to Penick. A two-stage regeneration system with catalyst cooling is described in U.S. Pat. No. 4,965,232 to Mauleon et al. where regenerated catalyst is removed from the second stage and sent to a holding vessel where it is then sent to an external heat exchanger and cooled catalyst is returned to the first stage of the regeneration zone.
Regulation of the amount of cooling in the heat exchangers is achieved in various ways. For instance, U.S. Pat. Nos. 4,434,245, 4,353,812 and 4,439,533 disclose hydrocarbon conversion processes wherein the catalyst is removed from a regenerator and cooled in side or external heat exchange coolers and then returned to the regenerator. The method described for controlling heat removal in the regenerator involves the extent of immersion of the cooling coils in the dense phase regenerated catalyst bed or controlling the rate of flow of regenerated catalyst through the external coolers. U.S. Pat. No. 2,436,927 discloses a fluidized catalytic conversion process wherein the crude feed is contacted with a silica-alumina type catalyst for producing high quality gasoline. Heat removal is achieved through the use of an external cooler and control is achieved by regulating the amount of catalyst passing through that cooler. U.S. Pat. Nos. 3,990,992 and 4,219,442 illustrate regenerator units having heat removal means different from those described above. Heat removal is achieved through internal coils in the upper section of the regenerator. Temperature control is achieved by controlling the amount of regenerated catalyst removed to the upper zone and then reintroduced along with coke-contaminated catalyst to the combustion zone. The balance of the regenerated catalyst is reintroduced to the catalytic reactor.
U.S. Pat. No. 5,351,749 to Lai et al., and U.S. Pat. No. 5,571,482 to Long et al., disclose catalyst cooler apparatuses comprising substantially vertical, cylindrical and close-ended heat removal vessels with one or more modulized heat exchange tube bundles. Fluidized solid particles flow downwardly in a dense phase fluidized bed, through the interior of the cooling vessel, contacting the one or more separate modular heat exchange tube units located therein. A coolant is passed through the heat exchange tube units, each of which comprises a coolant inlet tube, a discharge tube and a coolant collecting chamber where the vapor is generated, creating a closed type coolant-to-vapor circulation. Both systems disclose a gas distributor in flow communication with a fluidizing gas conduit at the lower portion of the heat exchange vessel to discharge fluidizing gas to maintain the fluidized state of the catalyst bed in the heat exchanger, enhance circulation and promote heat exchange. The amount of cooling achieved can be regulated by variation in the amount and rate of catalyst circulating through the cooling vessel and/or the amount and rate of fluidizing gas flowing through the cooler and/or the amount, rate and/or type of cooling liquid flowing through the cooler.
The above processes and systems, however, do not teach the apparatus and process discovered by the present inventor of reducing heat exposure during the catalyst stripping stage, nor do they suggest the advantages realized by the present invention. Moreover, in relation to the catalyst stripping phase, the prior art is replete with teachings of apparatus and processes for diverting additional heat to the stripping process to increase the temperature of the catalyst during the stripping phase, i.e., the opposite of cooling the catalyst during the stripping phase.
The practice of diverting heat to the catalyst stripping phase is thought to assist in removing hydrocarbons from the spent catalyst by vaporizing the higher boiling hydrocarbons from the surface of the catalyst. The commonly employed zeolite catalysts act as an adsorbent, absorbing hydrocarbons onto the surface of the catalyst. At temperatures below 950° F., stripping efficiency generally becomes diminished and these hydrocarbons are not removed and recovered, but rather are burned off in the regeneration phase.
The present inventor has discovered, however, that in high temperature cracking operations, heating the catalyst in the stripper is not the issue. Instead, in these high temperature cracking operations, the temperature in the stripper may reach levels above those necessary for relatively efficient stripping. The present inventor has recognized that, in fact, in these high temperature cracking operations, the stripper temperature combined with the presence of steam may lead to problems with catalyst deactivation. Thus, the present inventor has now discovered unexpectedly that by employing the present invention to maintain the spent catalyst temperature in the stripping zone to below about 1100° F., preferably below about 1050° F., provides increased catalyst life and improved efficiencies in catalyst performance.
The present invention is especially important for the high temperature cracking processes practiced currently to either crack heavier and heavier feeds or produce olefins. With regenerator temperatures today approaching 1700° F. and cracking conditions approaching upwards of 1500° F., it is more common for the catalyst to stay at relatively high deactivating temperatures in the presence of steam such that catalyst in the stripper zone may be damaged.
The inventors have discovered that by using the present process and an apparatus to reduce heat exposure to the catalyst during the stripping stage, reduction in catalyst deactivation and increased overall cracking efficiency are achieved.